1. Field of the Invention
The field of the invention is mechanical structures and particularly micro-machined mechanical structures, comprising a moving mass connected by at least one beam with two ends at an anchor point, one of the ends of the beam being connected to the moving mass and the other end being connected to the anchor point. It is also related to a device and particularly a sensor incorporating the structure.
2. State of the Art
Mechanical oscillators exist in which the force applied to a moving mass and the movement of this moving mass are related by a non-linear relation, and in this case the stiffness of a beam coupling the moving mass to a fixed structure of the oscillator is variable as a function of the displacement amplitude of the moving mass. This effect of the variation in the stiffness of the beam is more sensitive when the movement amplitude of the moving mass increases. The induced effect may be sub-linear or supra-linear. The non-linear relation between the force applied to the moving mass and the movement amplitude of the moving mass means that there can be two possible amplitudes of the movement at frequencies close to resonance for a given frequency. Therefore the movement becomes unstable. When a structure is excited close to its resonant frequency, the movement amplitude of the moving mass with respect to the static position of the moving mass is amplified by a factor called the xe2x80x9cQ quality factorxe2x80x9d. This factor is higher when energy losses in the mechanical structure are low. This amplification is used to obtain large oscillation amplitudes with low excitation forces. The mechanical transfer function (movement as a function of the excitation frequency) becomes asymmetric close to the resonant frequency and then becomes unstable. The non-linearity phenomenon of the relation between the force applied to the moving mass and the movement amplitude of the moving mass limits the amplitude of the movement that can be accepted if the movement is to be remain stable. For example, silicon micro-structures are observed with built in-built in type beams with a length of a few hundred xcexcm, for which the oscillation becomes unstable for movement amplitudes of a few xcexcm. This limits important performances for some systems, for example the sensitivity of a measurement device in which such a structure is used. In order to limit the non-linearity phenomenon, attempts have been made to limit the oscillation amplitude of the moving mass. Thus, the system remains within the linear range and a stable movement is possible. Thus, patent DE-202 445 2 B granted to IBM Corp. describes a monolithic electromechanical oscillator comprising a semi-conducting part for which the mechanical resonant frequency determines the oscillation frequency. An oscillation amplitude control circuit is integrated in the oscillator. The oscillation amplitude control circuit controls the excitation energy, in other words the current flux through a heating resistance as a function of a threshold value and the amplitudes of the observed real oscillation. Another example in which the oscillation amplitude is limited is described in patent SU 493 770 A awarded to KAUN POLY. In this patent, the vibration amplitude is captured. When the vibration amplitude exceeds a predetermined threshold, a means changes the stiffness of an elastic system such that the resonant frequency is changed and therefore the vibration amplitude is limited. Known examples of mechanical structures comprising an oscillating moving mass will now be described with reference to FIGS. 1A to 1D. Identical reference numbers in these figures denote elements with the same functions.
FIG. 1A represents a mechanical structure 1 incorporating a fixed frame 2 within which a moving mass 3 oscillates. The moving mass 3 is connected to the fixed frame 2 through beams 4 and 5. The movement direction represented by a double arrow 10 is perpendicular to beams 4, 5 and is located in the XOY plane in the figure. The movement of the moving mass is parallel to the OY direction. End 11 of the beam 4 is connected to the moving mass 3, and end 12 is connected to an anchor point 8 fixed in the OY direction of the movement of the moving mass. Similarly, end 13 of beam 5 is connected to the moving mass and end 14 is connected to an anchor point 9 fixed in the OY direction. FIG. 1B shows a mechanical structure 1 comprising a moving mass 3 as shown in FIG. 1A, but in the case in FIG. 1B, the moving mass 3 is connected through a set of four beams to the anchors 8 and 9 respectively, therefore there are two additional beams 6, 7 with ends 15, 16; 17, 18 respectively, these beams coupling the moving mass 3 to anchors 8 and 9 respectively. FIGS. 1C and 1D also show a mechanical structure 1 incorporating a moving mass 3 in which the beams connecting the moving mass 3 to anchor points 8, 9 respectively are not straight beams. The shape of the beams 4, 5 or 6, 7 shown in FIGS. 1C and 1D enables deformation of the beam in the XOY plane and consequently larger vibration amplitudes of the moving mass 3. This larger vibration amplitude of the moving mass 3 is made without non-linear phenomena occurring, precisely due to the shape of beams 4, 5, 6 or 7. This type of deformable beam, that for example can be found in patent application WO 95/34798 assigned to BOSCH, has the disadvantage that it has a moving mass 3 that oscillates not only in the Y direction in the XOY plane, but also in the X direction of the XOY plane. The result is parasite phenomena that disturb the signal that can be produced by such a device, and particularly a sensor with this type of mechanical structure. For example, these parasite phenomena can produce a shift in the resonant frequency by the occurrence of mechanical deformation modes that can be combined with the required excitation mode in the Y direction and finally by increased sensitivity to accelerations along several axes. In the latter case, the result is a reduced control over the directivity of the movement.
The problem of dependence between the vibration amplitude of the oscillating mass and the vibration frequency of this mass is discussed in patent U.S. Pat. No. 5,902,012 awarded to BOEING NORTH AMERICAN.
This patent (column 1, lines 44-48) describes that the vibration amplitude may be as high as 20% of the length of the suspension beams supporting the oscillating mass, and that the elongation of the beams in their axial direction can no longer be ignored under these conditions. To overcome this fact, this patent (column 1, line 63-column 2, line 2) proposes to make the suspension beam more easily extendible, and to do this by modifying the beam or the configuration of the frame or the mass at the location at which this suspension beam is attached.
This better extendibility is obtained either by:
forming each suspension beam with a curved shape in the plane of vibration of the oscillating mass as shown in FIG. 1 in this patent; or
by providing parts to enable relaxation of the elongation stress, for example in the form of cut-outs made at the connections between the beam and the fixed frame and/or the oscillating mass, or in the form of cut-outs made on the elongation beam as shown in FIGS. 2 to 5 in this patent.
Note that in all the examples given, the elongation means are symmetrical with respect to the stiffness. This means that the value of the resistance to elongation is the same for the same value of the tension applied along each direction of the axial line of the suspension beam. In other words, the deformation of the elongation means is the same regardless of whether the tension is made in one direction or in the opposite direction.
The invention is intended to provide a mechanical structure in which the oscillating moving mass moves along a known axis without moving in other directions, and according to a linear movement without any of the instabilities that can be observed when the movement is not linear. The invention is intended to offer this linear movement with a greater oscillation amplitude than the oscillation amplitude that could be obtained with mechanical structures according to prior art, for example as described in FIGS. 1A and 1B or in Boeing U.S. Pat. No. 5,920,012. Measurement sensors with improved performances can then be made due to conservation of movement linearity and a larger oscillation amplitude.
Finally, the invention is intended to provide a mechanical structure in which the movement of the vibrating mass is not sensitive or is only slightly sensitive to accelerations or shocks along an axis perpendicular to the movement direction of the moving mass.
As described above, it is known that linear mechanics are no longer applicable when the stiffness of the beams supporting the moving mass varies as a function of the displacement amplitude. This phenomenon is known in mechanics and for example is explained in GW Van SANTEL, xe2x80x9cvibration mxc3xa9caniquexe2x80x9d (mechanical vibration), Philipps technical library, Dunod Paris, 1957. Other references also describe this phenomenon such as Muck-G, Muller-G, Kupke-W, Nave-P, Seidel-H, xe2x80x9cObservation of non linear effect in the resonance behaviour of a micro-machined silicon accelerometerxe2x80x9d; and Pavena-R, Gotchev-D, xe2x80x9cNon linear vibration behaviour of thin multilayer diaphragmsxe2x80x9d. In order to obtain a linear movement, and therefore with no variation in the stiffness but in a single direction only, the inventors considered separating the suspension function from the stress relaxation function. According to the invention, the stress relaxation function is obtained by means distinct from the main beams such as 4, 5, 6 or 7 that support the suspension function. Therefore these means that support the stress relaxation function make it possible to increase the oscillation amplitude without introducing degrees of freedom that could modify the other properties, and particularly the oscillation frequency. Furthermore, these means make it possible to reduce the disturbance applied to the movement of the oscillating moving mass by an acceleration or a shock along a direction perpendicular to the movement direction of the moving mass. The principle of the invention is that the movement of the moving mass exerts an elongation or compression type stress on the beam(s) that connect the moving mass to the anchor points. This change in stresses causes a variation in the stiffness of the connecting beams. This phenomenon is not very visible in the case of free built in beams as long as the free end has the degree of freedom necessary to keep the stiffness of the beam(s) constant during the oscillation. However, it is very important in the frequent case of built in-built in beams like those shown in FIGS. 1A to 1B. The stress relaxation means according to the invention comprises at least one beam for which the geometric section, length and curvature are calculated so as to cancel the variation of the stiffness in a main beam during oscillation of the moving mass, and secondly to add asymmetry to the response of the stress relaxation means of the suspension beam. This means that, unlike elongation means described in patents U.S. Pat. No. 5,920,012 mentioned above, the apparent stiffness of the suspension means, including the suspension beam and its elongation means, will be asymmetric. This means that the apparent stiffness of the suspension means will be modified differently depending on the direction of the force exerted along the axial line of the suspension beam. In other words, the deformation of the elongation means will be different for two forces with the same absolute value but exerted in opposite directions. Ideally, the apparent variation of the stiffness will be zero when the force is applied in one direction and will be large when the same force is applied in the opposite direction.
Due to this asymmetry of the stiffness response of the elongation means, a sensor equipped with the invention may be made less sensitive to an acceleration along the axial line of the suspension means.
In one example embodiment that will be commented in more detail later, the stress relaxation means is in the form of a beam. This beam is fixed by means of an anchorage at two points. These two points define a straight line perpendicular to an axial direction of the suspension means. An axial line of the stress relaxation means is in the form of a curve that is symmetrical about the axial direction of the suspension means mechanically connected to this stress relaxation beam such that this curve is in the form of two half parts that are symmetrical to each other. This curve forms a hollow, in which the low point coincides with the junction point between the suspension beam and the stress relaxation beam. Each symmetrical half part has a point of inflection.
When the moving mass moves away from its rest position, the suspension means exerts tension on the stress relaxation beam. Since the shape of this beam forms a hollow and a double point of inflection under the effect of the tension applied to the bottom of the hollow, the beam tends to flatten and therefore reduce its length such that it is acting in compression. The result is that the stress relaxation beam according to the invention always works in compression during oscillating movements of the moving mass.
If the suspension beam is subjected to an acceleration with a component along the axial direction of the beam such that a force along this axial direction is exerted on the bottom of the stress relaxation beam approximately perpendicular to this beam, this force tends to increase the depth of the hollow formed by this beam, and this beam will act in tension. The inventors have noted that with this shape, the apparent stiffness of the stress relaxation beam acting in tension is greater than the stiffness of the same beam acting in compression.
The inventors make use of this asymmetry of the apparent stiffness depending on whether the stress relaxation beam is acting in tension or compression, to make the suspension insensitive or at least less sensitive to accelerations along the axial direction of the suspension beam acting in a direction tending to increase the depth of the hollow in the stress relaxation beam. If it is also required to make the suspension insensitive or at least less sensitive to accelerations also applied in a direction opposite to the first direction, the oscillating mass could be suspended by using two stress relaxation beams symmetric to each other about an axis perpendicular to the axial direction of the suspension means. The means that will subsequently be referred to as xe2x80x9celongation meansxe2x80x9d is connected to the main beam that forms a xe2x80x9csuspension elementxe2x80x9d, at at least one of its two ends, in which case it forms the link between the said suspension element and the anchor and/or the moving mass. The elongation means may also be connected to the suspension element at a partition of this element, for example if an elongation means is composed of several beams. According to the invention, the deformation of the elongation means will result in an elongation of the dimension of the main beam and the elongation means as a function of the amplitude of the oscillation. This deformation is applied under the influence of the tension applied by the main beam on the elongation means. This tension force is such that the stress exerted in the main beam remains approximately constant.
In summary, the invention is related to a mechanical structure building in a moving mass along an OY axis, this moving mass being suspended by suspension elements mechanically connected firstly to the moving mass and secondly to fixed anchor means, the structure comprising an elongation means mechanically connected to each suspension element inserted between the anchor means and the moving mass, this means forming an improved suspension means with the suspension element, with a first end connected to the anchor means and a second end connected to the moving mass, the elongation means being deformable in an XOY plane, the direction OX being the direction connecting the first end to the second end of the improved suspension means, structure characterised in that the stiffness of the elongation means is asymmetric, and with the improved suspension means an applied force causes a lower apparent stiffness variation when it is exerted in one direction along the OX axis than when the same force is applied in the opposite direction.
The shape of a mechanical means satisfying this asymmetry condition can be calculated using a digital simulation with the finite elements method, for example using the ANSYS software. The asymmetry condition may induce asymmetry in the shape of the mechanical means, or in a variation of its width or thickness or a combination of these three asymmetries.
According to one example embodiment that will be described briefly below and in more detail later, at least one of the elongation means is in the form of at least one beam with two ends and an axial line with a hollow with a bottom, this beam being symmetrical with respect to an axis of the suspension element that is connected to it, such that the said axial line is in the form of two half parts symmetrical to each other, each half part having a point of inflection.
Ideally, the deformation of the elongation means must be such that the stiffness of the improved suspension means in the suspension element remains constant. This means that the tension or compression force generated by the movement of the moving mass in the suspension element remains constant. There are at least two ends to the elongation means itself.
As described above, the suspension element is always mechanically coupled to the point of symmetry, which is therefore the mid-point of the suspension element.
An elongation means may be composed of one or several elongation beams, preferably identical and parallel to each other.
A suspension element may be coupled to an elongation beam at only one of its ends, in which case this elongation beam forms the elongation means, and the elongation means then has two ends that may be connected either to the anchor means or to the moving mass. A suspension element may also be coupled to an elongation beam at each of its ends, in which case these elongation beams form the entire elongation means. In this case, the two ends of a first elongation beam are mechanically connected to the anchor means and the two ends of the other elongation beam are connected to the moving mass.
Finally, the elongation means may comprise a first group of beams in which the hollows are oriented in the same direction.
Preferably, the beams in this group are identical and parallel to each other. Each beam in the group is connected to the suspension element, and the two ends of each of these beams in the first group are connected to the anchor means or to the oscillating mass.
In one advantageous embodiment of the invention, the elongation beam has a straight part forming the bottom of the hollow formed by the beam.